Generally, optical recording has advantages in that, e.g., a recording medium and a head do not contact each other when compared to magnetic recording, and high-density recording can be performed. Various types of media are known as optical recording media for optical recording, e.g., a read-only medium, a write-once medium, and an erasable programmable medium.
Optical recording media of these types have already been in practical use in various fields in the forms of, e.g., an optical recording disk (e.g., a CD, a CD-ROM, a CD-I, a laser disk, write-once and programmable disks, and a magneto-optical disk) and an optical card (e.g., a ROM card, write-once and programmable cards, and a magneto-optical card).
In a conventional read-only or write-once optical recording disk, a laser beam generated by a light source, e.g., a semiconductor laser, is focused to form a spot having a diameter of 1 to 2 .mu.m on the disk surface, and the surface state of the corresponding portion on the disk surface is detected by utilizing light reflected by the disk surface.
More specifically, when a pit is present, light is randomly reflected to reduce the intensity of the reflected light. When a pit is not present, light is reflected by a mirror surface. Data is read by utilizing a difference (intensity of light) between these two states.
In this case, however, a decrease in spot diameter of the laser light beam is limited to as small as about 1 to 2 .mu.m, and it is impossible to decrease the spot diameter to be extremely smaller than this.
Accordingly, the information amount of the conventional optical recording disk is limited by the fact that one pit on the disk represents one piece of information and by the limited spot diameter of the laser light beam. As a result, information recording at a high density exceeding a certain limit cannot be performed.